Demand for high-speed, large-capacity data transmission has also grown in wireless communications with rapid development of multimedia communications in recent years. Accordingly, a communication method for executing high-speed, large-capacity data transmission using frequencies efficiently in a limited frequency band is required. Then, attention is focused on a communication method for conducting communications using a plurality of antennas at both the transmission and reception parties. As a plurality of antennas are used for both the transmission and reception, if different signals are transmitted at the same frequency at the same time, appropriate processing is performed for the signals at the reception side or both the transmission and reception parties, the transmitted signals can be separated for reception. Accordingly, the transmission capacity can be increased without enlarging the frequency band and high-speed, large-capacity data transmission can be accomplished.
Proposed as the communication method as described above is an art wherein signals of a plurality of antennas are weighted according to a coefficient called weight at the transmission and reception parties, whereby if a plurality of signals are transmitted at the same time, the signals can be separated for reception (for example, refer to patent document 1). As a method of determining the weight in transmission and reception, a channel estimation value representing the communication channel situation is detected at the reception side and a characteristic vector is calculated from the channel estimation value and is used as weight. Since the channel estimation value is unknown at the transmission side, the channel estimation value detected at the reception side or the weight calculated at the reception side is fed back using a reverse communication link, whereby the weight at the transmission side is determined. In a time division duplex (TDD) system, the transmission side can use the reception channel estimation value on the reverse communication link. Since the same frequency is shared between the forward and reverse links, the communication channel situation can be assumed to be identical.
To thus share the channel estimation value between the forward and reverse links, preferably the transmission and reception circuits of the wireless communication apparatus are equal in amplitude and phase; in the actual apparatus, however, deviation occurs in the amplitude and the phase between the transmission and reception circuits because of the individual difference among radio frequency circuits of a power amplifier, etc., and transfer links, deviation of the circuit characteristic caused by temperature change of the environment, etc.
The channel estimation values detected at the forward and reverse reception parties when amplitude, phase deviation exists between the transmission and reception circuits of a wireless communication apparatus as described above are shown in Expressions (1) and (2) shown below. Here, let the forward channel estimation value be HFL, the reverse channel estimation value be HRL, and a channel response matrix that a wireless communication signal receives in propagation from a forward transmission antenna to reception antenna be H. Let amplitude, phase deviation of a transmission circuit and amplitude, phase deviation of a reception circuit in the forward direction be ZFL—TX and ZFL—RX respectively and amplitude, phase deviation of a transmission circuit and amplitude, phase deviation of a reception circuit in the reverse direction be ZRL—TX and ZRL—RX respectively. Let the number of forward transmission antennas be M and the number of reception antennas be N. T denotes transposition of matrix. Accordingly, HFL and HRL become as follows:
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                      H          FL                =                                            Z              FL_Rx                        ·            H            ·                          Z              FL_Tx                                ⁢                                          ⁢                                          =                                    [                                                                                          z                                              FL_Rx                        ⁢                                                                                                  ⁢                        1                                                                                                                                                                                                    0                                                                                                                                                                                          ⋱                                                                                                                                                                                          0                                                                                                                                                                                z                      FL_RxN                                                                                  ]                        ·                          [                                                          ⁢                                                                                          h                                              1                        ,                        1                                                                                                                        h                                              1                        ,                        2                                                                                                  …                                                                              h                                              1                        ,                        m                                                                                                                                  ⋮                                                                                                                                                                                h                                              i                        ,                        j                                                                                                  ⋮                                                                                                              h                                              N                        ,                        1                                                                                                                        h                                              N                        ,                        2                                                                                                  …                                                                              h                                              N                        ,                        M                                                                                                        ]                        ·                                                  ⁢                                                  ⁢                          [                                                          ⁢                                                                                          z                                              FL_Tx                        ⁢                                                                                                  ⁢                        1                                                                                                                                                                                                                                                                                                      0                                                                                                                                                                                                                z                                              FL_Tx                        ⁢                                                                                                  ⁢                        2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  z                      FL_TxM                                                                                  ⁢                                                          ]                                                          (        1        )                                [                  Expression          ⁢                                          ⁢          2                ]                                                                      H          RL                =                                            Z              RL_Rx                        ·            H            ·                          Z              RL_Tx                                ⁢                                          ⁢                                          =                                    [                                                          ⁢                                                                                          z                                              RL_Rx                        ⁢                                                                                                  ⁢                        1                                                                                                                                                                                                                                                                                                      0                                                                                                                                                                                                                z                                              RL_Rx                        ⁢                                                                                                  ⁢                        2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  z                      RL_RxM                                                                                  ⁢                                                          ]                        ·                          [                                                          ⁢                                                                                          h                                              1                        ,                        1                                                                                                  …                                                                              h                                              N                        ,                        1                                                                                                                                                        h                                              1                        ,                        2                                                                                                                                                                                                                          h                                              N                        ,                        2                                                                                                                                  ⋮                                                                              h                                              i                        ,                        j                                                                                                  ⋮                                                                                                              h                                              1                        ,                        M                                                                                                  …                                                                              h                                              N                        ,                        M                                                                                                        ]                        ·                                                  ⁢                                                  ⁢                          [                                                                                          z                                              RL_Tx                        ⁢                                                                                                  ⁢                        1                                                                                                                                                                                                    0                                                                                                                                                                                          ⋱                                                                                                                                                                                          0                                                                                                                                                                                z                      RL_TxN                                                                                  ]                                                          (        2        )            
Thus, if amplitude, phase deviation occurs in the transmission and reception circuits of the wireless communication apparatus, the forward and reverse channel estimation values differ. Thus, if the forward channel estimation value detected at the forward reception side is used to generate reception weight, no problem arises; however, if the forward channel estimation value is used to generate reverse transmission weight, effect occurs from amplitude, phase deviation between the transmission and reception circuits and the characteristic is degraded.
FIG. 8 shows an example of finding the reception characteristic (error rate) by computer simulation when amplitude, phase deviation occurs between the transmission and reception circuits of a wireless communication apparatus. The simulation conditions are as follows: Modulation system is QPSK; the number of transmission antennas is two; the number of reception antennas is two; the characteristic vectors of channel estimation values are used for transmission and reception weights; reception weight is generated using a forward channel estimation value; and transmission weight is generated using a reverse channel estimation value. Amplitude deviation 5% and phase deviation (excursion)±30, 60, and 90 degrees are generated as deviation between the transmission and reception circuits of the wireless communication apparatus. From FIG. 8, it can be acknowledged that the reception characteristic is degraded as the phase deviation (excursion) grows.
On the other hand, an art of detecting and correcting deviation of amplitude and phase occurring between transmission and reception circuits in a wireless communication apparatus is proposed (for example, refer to patent document 2). The wireless apparatus is configured to detect deviation occurring between the transmission and reception circuits using a communication signal used for transmission generated by the wireless communication apparatus and a reference signal not used as the communication signal, and has an antenna and the transmission circuit and the reception circuit sharing the antenna. At the calibration time, output of the transmission circuit is connected to input of the reception circuit and the phase rotation amount and/or the amplitude deviation amount of a signal passing through the transmission and reception circuits is calculated. Next, the reference signal is connected to the input of the reception circuit and the phase rotation amount and/or the amplitude deviation amount of signal passing through the reception circuit is calculated. The information pieces are subtracted, whereby the phase rotation amount and/or the amplitude deviation amount of the signal passing through the transmission circuit is calculated. Based on the information, a correction value for correcting the phase rotation amount and/or the amplitude deviation amount difference between the transmission and reception circuits is calculated. Using the correction value, the phase rotation amount and/or the amplitude deviation amount difference between the transmission and reception circuits is corrected.    Patent document 1: JP-A-2001-237751    Patent document 2: International Patent Publication No. 00/60757